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Cilia and Flagella

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com/watch?v=2lA2faVXt7A
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com/watch?v=9nZYlyFGm50

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Motile cilia
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Tiny, hairlike, slender appendages about 0
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2 μm in size
Latin Cili = eyelash
Full of cytosol all the way to their tips
Covered by membrane continuous with plasma membrane
Contain a bundle of MTs at their core, called axoneme
Extend (project) from the surface of many kinds of cells
Found in most animal species, many protozoa, and some lower plants (occur in cycads)
Present in a few types of specialized cells such as the epithelial cells lining the respiratory tract,
epithelial cells lining the gastrointestinal tract, fallopian tubes

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• They are motile and designed to
• Move the cell itself
• Move substances over and around the cell
• Cilia move by the bending of an axoneme - a complex bundle of MTs
• Use the ATP in its cytosol to generates force all the way along their length
• Grow from basal bodies that are closely related to centrioles
• If cilia are sheared from the cell, they rapidly reform by elongating from structures called basal
bodies
• Cilia are more in number and smaller in contrast to flagella

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Flagella
• Discovered by Engelman in 1868; Structure first studied by Jansen in 1887
• Latin word meaning ‘little whip’
• Projection from the cell
• Internal structure is similar to cilia
• Made up of MTs forming axoneme
• Full of cytosol all the way to their tips
• Covered by membrane continuous with plasma membrane
• Contain a bundle of MTs at their core, called axoneme

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• Use the ATP in its cytosol to generate force all the way along their length
• Bacterial flagella is entirely driven by the rotary motor at its base that functions through PMF
• They are motile and designed to
• Move the cell itself
• Move substances over and around the cell
• The movement of a flagellum is produced by the bending of its axoneme

• Grow from basal bodies that are closely related to centrioles
• If flagella are sheared from the cell, they rapidly reform by elongating from structures called basal
bodies
• Flagella are lesser in number and larger in contrast to cilia (up to 1000 μm)
• They exist in motile cells such as the male gamete spermatozoon

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Basal body / Blepharoplast / Kinetosome / Cell body at the base of
cilia and flagella

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Basal body plays an important role in initiating the growth of axoneme
Present at the base of cilia / flagella
Derived from centriole – structurally identical to centriole
Act as MTOC for ciliary / flagellar MTs
At its point of attachment of cell, the axoneme connects with the basal body
Cylindrical hollow structures about 0
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4 μ long

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• Structure similar to centriole
• Contains nine triplet MTs – A, B, C
• Nine sets of three MTs, fused into triplets, form the wall of the basal body
• ‘A’ MT - complete with 13 protofilaments, fused to incomplete B microtubule
• ‘B’ MT – incomplete, shares protofilaments from A, fused to incomplete C
• ‘C’ MT – incomplete, shares protofilaments from B
• ‘A’ and ‘B’ MTs of basal bodies continue into the axonemal shaft
• ‘C’ MT terminates within the transition zone between the basal body and the shaft
• Each triplet is tilted inward like the blades of a turbine
• Adjacent triplets are linked at intervals along their length by AC linkers
• Protein spokes radiate out to each triplet from a central core, forming a pattern like a cartwheel

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• Basal body forms the lower portion of a ciliary axoneme, and it is composed of nine sets of triplet
MTs, each triplet containing one complete MT (the A tubule) fused to two incomplete MTs (the B and
C tubules)
• Proteins [shown in red in (B)] form links that hold the cylindrical array of MTs together

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• Cilia and flagella grow from basal bodies that are closely related to centrioles
• If cilia or flagella are sheared from the cell, they rapidly reform by elongating from basal bodies
• During the formation or regeneration of a cilium, each doublet MT of the axoneme grows from
two of the MTs in the triplet MTs of the basal body
• Nine-fold symmetry of the basal body MTs is preserved in the axoneme
• Addition of tubulin and other proteins of the axoneme takes place at the distal tip of the
structure, at the plus end of the MTs

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Ciliary or flagellar axoneme

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9+2

9+0

Relationship between MT structure of basal body and axoneme of cilia or flagella
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Axoneme of motile cilia and flagella (9 + 2 axoneme)
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Cilia and flagella have similar organization in all eukaryotic cells
Possesses a central bundle of MTs, called axoneme
~10 μ long; may be as long as 200 μ in some cells
Composed entirely of MTs and their associated proteins
MTs extends continuously for the length of the axoneme
Surrounded by the plasma membrane

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With rare exceptions, cilia and flagella have 9 + 2 axoneme from protozoa to mammals
9-fold symmetry
MTs are modified and arranged in a special pattern
MTs in axoneme are in same polarity, their plus ends are at the tip and minus end at the base
9 outer doublet MTs (Outer A-B doublets) arranged in a ring around a pair of single MTs (two
singlet MTs or central pair)
• Central tubules are enclosed in central sheath, which is connected to ‘A’ tubules of outer doublets
• This "9 + 2" array (or 9 x 2 + 2) is characteristic of almost all forms of cilia and eukaryotic flagella

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• Each member of the pair of MTs is a complete MT
• Each of the outer doublets is composed of one complete (‘A’) and one partial (‘B’) MT fused
together so that they share a common tubule wall

• Complete MTs of the outer doublet (A) as well as two singlets are formed from a ring of 13 subunits
• Incomplete MTs of the outer doublet (B and C) is formed from only 11 subunits, sharing subunits
with the neighbours

• ‘A’ and ‘B’ MTs of axonemal shaft and basal bodies are continuous
• ‘C’ MT terminates within the transition zone between the basal body and the shaft
• Two central singlet MTs in cilium and flagellum also end in the transition zone, above the basal
body

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• MTs of an axoneme are associated with numerous proteins
• Axonemal cytoskeleton acts as scaffolding for various protein complexes and provides binding sites
for molecular motor proteins such as kinesin II, ciliary dynein

• These proteins project at regular positions along the length of the MTs
• Functions of different associated proteins
• Serve as cross-links that hold the bundle of MTs together
• Generate the force that drives the bending motion
• Form a mechanically activated relay system that controls the motion to produce the desired
waveform

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• Ciliary or flagellar dyneins
• Have two arms
• Outer arms (three-headed)
• Inner arms (two headed)
• Permanently attached to ‘A’ MTs (complete MT) in the outer doublets
• Project from ‘A’ MT
• Their globular heads reach out to the ‘B’ MT of the neighbouring MT doublet
• Extend from the doublets and join neighbouring doublets
• Equidistantly placed throughout the length of axoneme
• Spaced at 24 nm intervals
• Exert force on neighbouring MT doublet (see later)

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• Radial spokes
• Radiate from central singlets to each ‘A’ tubule of the outer doublets
• Join central sheath with the ‘A’ MTs of the outer doublets
• Emerge in groups of three, which repeat along the length of axoneme
• Head of the spoke faces inwards
• Nexin linkers
• Nexin is elastic protein that forms flexible linkers
• Crosslinking proteins – forms interdoublet bridge
• Link neighbouring outer MT doublets
• Attached to ‘A’ MTs of outer doublets that links to ‘B’ MTs of neighbouring outer doublet
• Spaced every 86 nm along the axoneme
• Part of a dynein regulatory complex
• Limits extent that adjacent doublets can slide over one another due to force exerted by dynein

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• Tektin
• Strengthens the junction between ‘A’ and ‘B’ MTs of one doublet
• A highly α-helical protein that is similar in structure to IF proteins
• Each tektin filament is 2 nm in diameter and ~48 nm long
• Tektin filament runs longitudinally along the wall of the outer doublet where the ‘A’ MT is joined
to the ‘B’ MT

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Cross-sectional view of a typical cilium /
flagellum showing the major structures
• Dynein arms and radial spokes with
attached heads surround a central pair
of singlet MTs

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Cross-section

Longitudinal-sections

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Movement of cilia and flagella

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Movement of cilia
• Fields of cilia bend in coordinated unidirectional waves
• Each cilium moves with a whiplike motion:
• A forward active stroke
• In which the cilium is fully extended and beating against the surrounding liquid
• A recovery phase
• In which the cilium returns to its original position with an unrolling movement that minimizes
viscous drag
• The cycles of adjacent cilia are almost but not quite in synchrony, creating the wavelike patterns that
can be seen in fields of beating cilia under the microscope

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Movement of flagella
• The simple flagella of sperm and of many protozoa are much like cilia in their internal structure,
but they are usually very much longer
• Instead of making whiplike movements, they show propellor like motion
• Nevertheless, the molecular basis for their movement is the same as that in cilia

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The contrasting motions of beating cilia and flagella
(A) The beat of a cilium such as that on an epithelial cell from the
human respiratory tract resembles the breast stroke in swimming
• A fast power stroke (stages 1 and 2), in which fluid is driven
over the surface of the cell, is followed by a slow recovery
stroke (stages 3, 4, and 5)
• Each cycle typically requires 0
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2 second and generates
a force perpendicular to the axis of the axoneme
(B) The wavelike movements of the flagellum of a sperm cell
• Waves of constant amplitude move continuously from the base
to the tip of a flagellum
• The cell is thereby pushed forward, a distinctly different effect
from that caused by a cilium

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Ciliary / flagellar dynein in axoneme drives the movements of cilia and flagella
• The most important of the accessory proteins is ciliary / flagellar dynein
• Heads of dynein interact with adjacent MTs to generate a sliding force between the MTs
• Because of the multiple links that hold adjacent MT doublets together, what would be a sliding
movement between free MTs is converted to a bending motion
• Like cytoplasmic dynein, ciliary / flagellar dynein has
• A motor domain, which hydrolyzes ATP to move along a MT toward its minus end
• A tail region that carries a cargo, which in this case is an adjacent MT
• Tail of ciliary / flagellar dynein binds only to the A tubule and not to the B tubule, which has a
slightly different structure

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Mechanism of ciliary and flagellar locomotion
Action of dynein
Dynein arms anchored to the ‘A’ tubule of lower doublet attach to the binding sites on the ‘B’ tubule of
neighbouring upper doublet
↓
Dynein molecules undergo a conformational change upon ATP hydrolysis, or power stroke, that
causes the lower doublet to slide toward the basal end of the upper doublet
↓
Dynein arms detach from the ‘B’ tubule of upper doublet
↓
Arms are reattached to the upper doublet so that another cycle can begin

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• Sliding filament model of bending
• Molecules of ciliary / flagellar dynein form bridges between the circumference of the axoneme
• Dynein arms have ATPase activity
• It converts the energy released by ATP hydrolysis into the mechanical work of ciliary and
flagellar beating
• When the motor domain of dynein is activated, the dynein molecules attached to the MT doublet
attempt to walk along the adjacent MT doublet
• In the presence of ATP, they can move from one tubulin to another
• Dynein thus enable the MTs to slide along one another

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• Dynein bridges are regulated so that sliding leads to synchronized bending
• Doublets are held in place due to nexin and radial spokes
• Hence sliding is limited lengthwise
• The presence of nexin between MT doublets prevents this sliding and the dynein force is
instead converted into a bending motion
• If nexin and radial spokes are subjected to enzyme digestion, and exposed to ATP, the
doublets will continue to slide and telescope up to 9X their length

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• Sliding on one side of the axoneme alternates with sliding on the other side
• Hence a part of the cilium or flagellum bends first in one direction and then in opposite
direction
• This requires that, at any given time, the dynein arms on one side of the axoneme are active while
those on the other side are inactive
• Dynein arms on one side of the axoneme are active at any given time
• Dynein powers the sliding of the MTs against one another, first on one side, then on the
other side
• As a result of this difference in dynein activity, the doublets on the inner side of the bend extend
beyond those on the opposite side of the axoneme

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Beating of cilia and wavelike motion of flagella
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Bending of an axoneme
(A)The sliding of outer MT doublets against each other causes the axoneme to elongate if the
proteins that link the doublets together are not present
(B) If the doublets are tied to each other at one end, the axoneme bends
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Intraflagellar (IFT) / Intraciliary (ICT) transport

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Intraflagellar / intraciliary transport (IFT / ICT)
• Bidirectional motility called IFT / ICT plays as essential
role to move the building materials (tubulin) from cell
body to the assembly site and disassembled material to
be recycled from tip back to cell body
• By regulating the equilibrium between these two
processes, the length of flagella / cilia can be
maintained dynamically
• Motor proteins help in IFT / ICT
• Kinesin and Cytoplasmic dynein
• Outward transport = Anterograde transport
• Inward transport = Retrograde transport
• Action already studied in MTs

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Functions of cilia and flagella

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Functions of motile cilia
• Move fluid over the surface of the cell
• Propel single cells through a fluid
• e
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, protozoa, for example, Paramecium, use cilia both to collect food particles and for
locomotion
• On the epithelial cells lining the human respiratory tract, huge numbers of cilia sweep layers of
mucus, together with trapped particles of dust and dead cells, up toward the mouth, where they are
swallowed and eliminated
• Help to sweep eggs along the oviduct
• In the gastrointestinal tract, the beating cilia aid digestion by mixing the ingested food with the
digestive secretions
• In fallopian tubes, beating of cilia moves ovum from ovary to uterus
Functions of non-motile cilia (primary cilia) (9 + 0 or 9 x 2 + 0 axoneme)
• Specialized primary cilia are found in human sensory organs such as eye and nose
• Have role in chemical / thermal / mechanical sensation
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Functions of flagella
• Locomotion of many protozoa, algae
• Rotation of the flagellum propels the sperm forward in a swimming motion that enables the sperm
to swim up the female reproductive tract

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